1. Chapter 14 The Chemistry of Alkynes

2. Alkynes Also known as “Acetylenes” Naturally occurring alkynes are relatively rare They do not occur as a petroleum constituents 2 14.10 Occurrence and Use of Alkynes Carotatoxin

3. Uses of Alkynes Acetylene is a chemical feedstock for many important compounds Occur in antiviral and antifungal drugs – Efavirenz 3 14.10 Occurrence and Use of Alkynes

4. Common Nomenclature Simple alkynes are named as acetylene derivatives 4 14.1 Nomenclature of Alkynes Certain compounds are derivatives of the propargyl group (HC  C-CH 2 -) acetylene

5. IUPAC Nomenclature Follow the rules for naming alkenes except, change “-ane” to “-yne” 5

6. IUPAC Nomenclature Principal groups that have priority over the triple bond receive preference in numbering and suffix – Recall: Carboxylic acid > anhydride > ester > acid halide amide > nitrile > aldehyde > ketone > alcohol > thiol > amine – Substituent groups containing a triple bond = alkynyl groups Named by replacing final “-e” with “-yl” 6 14.1 Nomenclature of Alkynes

7. Substitutive Nomenclature If both C=C and C  C are present, the bond with the lowest number gets precedence However, if the rule is ambiguous, the double bond gets precedence 7 14.1 Nomenclature of Alkynes

8. Problems Name the following compounds: 8

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10. Structure and Bonding Alkynes display linear geometry cis-trans isomerism cannot occur in alkynes Cycloalkynes smaller than cyclooctyne cannot be isolated under ordinary conditions 10 14.2 Structure and Bonding in Alkynes

11. MO Bonding Picture Alkynes utilize sp hybrid orbitals Electrons in an sp hybrid orbital are held closer to the nucleus on average 11 14.2 Structure and Bonding in Alkynes

12. MO Bonding Picture 12 14.2 Structure and Bonding in Alkynes

13. MO Bonding Picture 13 14.2 Structure and Bonding in Alkynes

14. Heats of Formation Alkynes are less stable than isomeric dienes Internal alkynes are more stable than terminal alkynes 14 14.2 Structure and Bonding in Alkynes

15. Boiling Points and Solubilities Alkyne boiling points are not much different from those of analogous alkenes and alkanes Similarly, alkynes have low densities and are insoluble in water 15 14.3 Physical Properties of Alkynes

16. IR Spectroscopy of Alkynes C  C stretch: 2100-2200 cm -1 Symmetrical alkynes will not show this stretch  C-H stretch: 3300 cm -1 16 14.3 Physical Properties of Alkynes

17. NMR Spectroscopy of Alkynes The reason for the unusual acetylenic proton chemical shift is similar to that described for vinylic protons 17 14.3 Physical Properties of Alkynes

18. NMR Spectroscopy of Alkynes However, the effect is in the opposite direction 18 14.3 Physical Properties of Alkynes

19. NMR Spectroscopy of Alkynes Alkynyl carbons typically appear at  65-80 Propargylic carbons also display smaller chemical shifts (5-10 ppm lower than alkyl) 19 14.3 Physical Properties of Alkynes

20. Preparation of Alkynes Alkynes prepared by elimination of HX from alkyl halides 20

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22. Addition Rxns of Alkynes Similar to addition reactions for alkenes – Alkyne addition rxns are often slower than correspondig alkene additions – Regioselectivity follows a similar course Addition of HX and X 2 : 22

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24. Problems Give the alkene products for the following addition rxns: 24

25. Regioselectivity of Second Addition Rxn 25

26. 26 14.4 Introduction to Addition Reactions of the Triple Bond Problems

27. Hydration of Alkynes Addition of H 2 O As with alkenes, alkynes can be hydrated by two methods 1)Mercury (II) Catalyzed Hydration The product is a ketone and not an alcohol 27 Recall:

28. Keto-Enol Tautomerism Tautomers: constitutional isomers that interconvert rapidly – Equilibrium lies far to the right 28

29. Mechanism of Mercury (II) Catalyzed Hydration 29 14.5 Conversion of Alkynes into Aldehydes and Ketones

30. Mechanism of Hydration 30 14.5 Conversion of Alkynes into Aldehydes and Ketones

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32. Problems 1)Give the complete mechanism for the following reaction: 2)What is the product for the following rxn? 32